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I was startled when I hooked up a DX-394 radio (RX2) to the output of the
2nd mixer of another DX-394 (RX1) to find that the strong MW station RX1 was tuned to showed up not only at the 2nd IF of 455 kHz but at every 5kHz from around 200kHz to 30MHz! The strength varied with frequency, probably in part with the gain vs frequency curve of RX2. There were stronger signals at the 2nd and 3rd harmonics of 455kHz. Had to reduce the RX1's RF gain to the point where its AGC was inactive and the receiver was operating just above its threshold of detection to diminish the forest to just a few trees. The 2nd mixer is preceded by a crystal filter at the 1st IF of 45MHz +/-2.5kHz so I expected to see spectrum of about 15-20kHz width across 455kHz and some 3rd and higher order products like 910 and 1365kHz. The 5kHz interval is suspiciously the same as the steps in the 1st LO VCO frequency (45.150 to 74.995 MHz), controlled by a digital PLL. The 2nd LO at 44.545MHz +/-2.5khz is a varactor tuned VCO. To generate products every 5kHz, surely we need a 5kHz signal or two very strong signals that are 5kHz apart going into the mixer or some non-linear stage after the 1st IF filter. Am I seeing something unusual or is this what I should see? If it's an anomaly, can anyone suggest what the cause might be? I'm thinking this is a fault or a design weakness that with more complex signals gives rise to a lot of interference to good reception. Could it be possible that this is the result of the method of observation and is not really occurring in normal practise? 73 Tom |
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Mixer products every 5kHz to 30MHz on DX-394? | Homebrew |